This invention provides an instrument for testing paramagnetic and diamagnetic metallic materials, as well as ferromagnetic materials above their Curie points, for material defects and dimensional accuracy. It is particularly useful for testing tubes and slabs above 1000.degree. C. The instrument essentially comprises an electromagnetic transducer (EMT) having two magnetic pole shoes, an eddy current exciting winding, a receiving winding, and an electronic signal processing unit for processing signals from the receiving winding.
In the pursuit of quality control and optimum production efficiency, it is advantageous to test the quality of a product at an early stage of its manufacture so that, in the event that a defect is detected, correction can be made and product will not be wasted. For example, in the manufacture of seamless tubes, especially steel tubes, it is important to test tolerance accuracy while the tube is hot in order to detect erroneous settings of production line equipment while there is still sufficient time to control the roll stands further down the line.
From literature on this subject, it is apparent that many different techniques are known for measuring hot test pieces. These known techniques can be classified into contacting measuring techniques and non-contacting measuring techniques.
One such technique uses an ultrasonic piezoelectric oscillator with a high-pressure flow water coupling. At high pressure a stretch of water is generated between the test piece and the ultrasonic oscillator. The test results achieved with such an arrangement are highly dependent upon the surface quality of the test piece. The test procedure is relatively slow and requires a relatively large quantity of water. Furthermore, the coupling water causes local cooling of the test piece which can cause strains and cracks in the test piece.
Another technique uses piezoelectric oscillators with pressure couplings. Such arrangements permit the direct coupling of the oscillator at high pressure within the range of a roll body. However, such arrangements limit the positioning of the measuring apparatus since measurements cannot be made at any desired point of the train of rollers. Also, it is impossible to take a measurement for tolerance accuracy.
It is also known to excite ultrasonic pulses by using a high capacity pulse laser to locally heat the test piece. However, the use of this instrument for industrial testing is limited.
In addition to the ultrasonic arrangements, it is also known to test materials in production using radiological methods. These methods require working with X-rays, beta rays or gamma rays and therefore present problems of protection from the radiation. Also, radiological testing requires a relatively long measuring time to obtain the accuracy of measurement normally required.
Another known technique is the electrodynamic excitation of ultrasonics. This technique is based on the principle that a force is exerted on a current-carrying body in a magnetic field, acting vertically on the plane that is charged by the magnetic field vector and the current vector. A solenoid generates the magnetic field via a pole shoe configuration while an eddy current pulse is excited in the test piece using a transformer coil. The intensity of the power pulse generated in such a manner and thus the force of the ultrasonic pulse is a function of the magnitude of the eddy current and on the strength of the magnetic field.
In order to be able to generate a sufficiently strong magnetic field and a large eddy current to permit ultrasonic evaluation of the test piece, the EMT head must be positioned very close to the test piece. Depending upon the design of the EMT head itself, different types of ultrasonic waves can be generated. Know methods and instruments are described in the German specification No. 26 55 804 and German application No. 26 21 684 and No. 28 43 804. However, these known instruments have an operational disadvantage. Dependng on its specific design, an EMT head is suitable only for generating transverse waves, guided waves or Lamb-waves, which at high temperatures in the test piece are strongly damped since the shearing module is highly decreasing and thus they cannot be exploited technically. In addition thereto, all of the abovementioned arrangements have the disadvantages that they are restricted to the use of ferritic materials or to such geometries of test pieces that fit between the pole shoes of the EMT. This means that the thickness of the test piece is limited to some millimeters.
The longitudinal wave transducer required for measurements taken on hot test pieces or such made of paramagnetic or diamagnetic metals are described, e.g., in the British Journal of Non-Destructive Testing, 20, 1978, No. 5, September, pages 242 to 247, in Material Evaluation, 34, 1976, No. 4, April pages 81 to 90; and in Ultrasonics, 16, 1978, No. 7, July, pages 151 to 155. However, these publications describe only transducers that are not suitable for use at high temperatures and within small distances of the test piece, as they permit only a maximum test piece temperature of 1000.degree. C.